Update internal usage to use the `git_reference` names for constants.

For testing, we may wish to use a man-in-the-middle proxy that can
inspect the CONNECT traffic to our test endpoints.  For this, we will
need to accept the proxy's certificate, which will not be valid for the
true endpoint.

Add a new environment variable, GITTEST_REMOTE_SSL_NOVERIFY to disable
https certificate validation for the tests.

Rename credential callback to proxy_cred_cb to match new cert callback.

Give the proxy tests a proxy certificate callback, and allow self-signed
certificates when the `GITTEST_REMOTE_PROXY_SELFSIGNED` environment
variable is set (to anything).  In that case, simply compare the hostname
from the callback to the hostname that we connected to.

As we want to support HTTPS proxies, support an optional
`GITTEST_REMOTE_PROXY_SCHEME` environment variable for tests that will
allow for HTTPS support.  (When unset, the tests default to HTTP
proxies.)

Change the `GITTEST_REMOTE_PROXY_URL` environment variable to be
`GITTEST_REMOTE_PROXY_HOST`, since it is a host:port combination, not an
actual URL.  (We cannot use a URL here since we may want to include the
username:password combination in the constructed URL.)

Before resetting the url and username, ensure that we free them in case
they were set by environment variables.

Don't just free the spec vector, also free the specs themselves.

Don't just free the push status structure, actually free the strings that were
strdup'd into the struct as well.